This will be a prospective, two group, time series, quasi- experimental study of the natural history of human immunodeficiency virus (HIV) infection in infants of drug addicted mothers. The specific aims of this study are to: 1) measure the immunologic aberrations that result from HIV infection, maternal drug addiction and the confluence of these variables, 2) determine the vertical transmission rate and timing (transplacental versus natal) of infection, 3) identify maternal characteristics that are associated with the highest frequency of HIV transmission and 4) identify indicators that re prognostic of progressive disease at a time when children are clinically well. This information is needed to define the preclinical immunologic patterns that result from HIV infection and differentiate those that result from other factors. Information on the rate and timing of HIV transmission will guide maternal counselling regarding family planning, as well as direct potential intervention strategies, that could ultimately result in decreased transmission to children. Accurate identification of HIV infection depends on new strategies in the first 12 months of life when serologic diagnosis is confounded by passive maternal antibody. Identifying transmission risks and early indicators of prognosis will lead to better preventive and therapeutic strategies. Over the 5 year study we will enroll 200 drug addicted mothers and their infants and 30 HIV infected mothers (and their infants) who have sexual contact as their only risk. After informed consent is obtained subjects will be interviewed about drug use and other risk factors and blood samples will be obtained (15 ml in mothers and 4 ml in infants and children). Data will be coded without direct identifiers. Blood will be tested for HIV by culture using reverse transcriptase and antigen detection; for HIV antigen using antigen capture methods, HIV antibody studies using ELISA, Western Blot, anti p121 and p24 IgG subclass assay using a dot- blot ELISA, HIV specific IgM using immunofluorescence, T lymphocyte typing, and specific cell mediated and humoral responses to antigens and mitogens (CMI). Data will be entered in our NCR PC6 computer and analyzed using log linear model analysis or logistic regression. Study parameters will be analyzed with reference to clinical course of patients to determine prognosis. Confidentiality of subjects will be preserved; counselling about results and meaning of HIV tests will be provided.